feat(web): support a reverse-proxy base URL (--base-url / CODEMAN_BASE_URL)

Codeman can now be mounted under a sub-path behind a reverse proxy that
forwards the prefix unchanged (e.g. https://host/codeman/). Default is `/`
(root), which is byte-identical to the historical behavior.

Design — few choke points, mirrored ingress/egress:
- src/config/base-path.ts: pure single-source normalize/validate/join/strip.
- Server ingress: stripBasePath() inside Fastify rewriteUrl, so routes stay
  declared prefix-agnostic; un-prefixed requests (hooks, health, docker bridge
  hitting the raw port) pass through unchanged.
- Server egress: one onSend hook prepends the base to root-absolute Location
  headers (covers all redirects).
- HTML: renderIndexHtml points <base href> at the mount and injects
  window.__CODEMAN_BASE__ — ONLY when a base is set (inert at root).
- Frontend runtime URLs: CodemanBase.url() route builder in constants.js,
  applied transparently by a fetch wrapper and explicitly at the
  EventSource/WebSocket/window.open/<img|iframe|a>-src sites.
- sw.js derives its base from self.location; manifest uses relative start_url/scope.
- Web-tab proxy: proxyPrefixFor(cap, basePath) is the single base-aware root that
  cascades to the injected <base>, HTML/attr rewrites, runtimeUrlShim, Set-Cookie
  Path and Location; capabilityFromReferer strips the base off the browser Referer,
  while the ingress parsers stay base-agnostic (rewriteUrl already stripped it).

--base-url rides the daemon relaunch (buildWebArgs) and the service unit
(resolveServicePlan). constants.js is guarded against a missing `window` for
isolated unit-test contexts.

Tests: test/base-path.test.ts (pure helpers), base-path coverage in
webview-proxy/render-index-html/daemon-control; CodemanBase stubbed in the
vm-isolated panels-ui test contexts. Docs: Remote-Access.md (sub-path section +
nginx example), security-architecture.md env table, CLAUDE.md pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XUkPBxbumnct6qSrx4JDju

src/web/public/session-ui.js

@@ -3046,7 +3046,7 @@ Object.assign(CodemanApp.prototype, {
           return `<div class="case-manage-item" style="display:flex; align-items:center; gap:8px; justify-content:space-between;">
             <span style="overflow:hidden; text-overflow:ellipsis; white-space:nowrap;" title="${nm}">${nm} <span class="form-hint">(${mb} MB)</span></span>
             <span style="flex-shrink:0;">
-              <a class="btn-toolbar" href="/api/docker-exports/${encodeURIComponent(e.name)}" download>Download</a>
+              <a class="btn-toolbar" href="${CodemanBase.url(`/api/docker-exports/${encodeURIComponent(e.name)}`)}" download>Download</a>
               <button class="btn-toolbar" onclick="app.importDockerBundle('${nm.replace(/'/g, "\\'")}')">Import</button>
               <button class="btn-toolbar" onclick="app.deleteDockerExport('${nm.replace(/'/g, "\\'")}')">Delete</button>
             </span>